1. Field of the Invention
The present invention relates to a radiation image information reading apparatus for reading radiation image information from a radiation conversion panel that is irradiated with a radiation, and a method of correcting the sensitivity of a radiation conversion panel.
2. Description of the Related Art
There is known a radiation image information recording and reading system which employs a stimulable phosphor which, when exposed to an applied radiation (X-rays, α-rays, β-rays, γ-rays, electron beams, ultraviolet radiation, or the like), stores a part of the energy of the radiation, and, when subsequently exposed to applied stimulating light such as visible light, emits light in proportion to the intensity of the stored energy of the radiation. In the radiation image information recording and reading system, the radiation image information of a subject such as a human body or the like is temporarily recorded in a radiation conversion panel having a stimulable phosphor sheet which comprises a stimulable phosphor layer in the form of a sheet, and then stimulating light such as a laser beam is applied to the radiation conversion panel to cause the radiation conversion panel to emit light representing the recorded radiation image information. The emitted light is photoelectrically read to produce an electric image signal. Based on the image signal, the radiation image information of the subject is outputted as a visible image on a recording medium such as a photographic photosensitive medium or the like or a display unit such as a CRT (Cathode Ray Tube) or the like. For details of the radiation image information recording and reading system, reference should be made to Japanese laid-open patent publication No. 55-12429.
The radiation conversion panel which is used in the radiation image information recording and reading system may comprise a scintillator for converting the energy of an applied radiation into visible light without temporarily storing the energy, rather than a stimulable phosphor sheet. Alternatively, the radiation conversion panel may comprise a solid-state radiation detector such as a thin-film transistor for converting the energy of an applied radiation directly into an electric signal.
The radiation conversion panels have their sensitivity lowered with the time that has elapsed from the shipment from the factory, i.e., the time for which they have been used, the number of times that they have been irradiated with a radiation, i.e., the number of shots taken thereby, or the irradiation dose accumulated thereon. Particularly, the radiation is not applied uniformly over the entire surface of the radiation conversion panel, but the irradiation dose thereof differs between a central area of the radiation conversion panel where the subject to be imaged is located most frequently and a peripheral area of the radiation conversion panel where the subject to be imaged is located less frequently. Furthermore, if the radiation is applied to the radiation conversion panel through an irradiation field stop, then any sensitivity drop is reduced in the peripheral area of the radiation conversion panel.
When radiation image information is recorded on or read from a radiation conversion panel whose sensitivity is lowered, the radiation conversion panel may fail to provide desired radiation image information due to sensitivity irregularities.